The present invention relates to an optical pickup device for reproduction of information recorded on an optical recording medium such as a video disc and a compact disc.
A moving magnet type (MM type) optical pickup is well known. The MM type includes a holder, an objective lens, a magnet and a coil. The magnet is fixedly attached to the movable holder which mounts thereon the objective lens, and the coil is fixedly and stationarily disposed in confronting relation with respect to the magnet. The magnet is moved by a force induced by a current flowing through the coil, so moving the holder, so that angular orientation or posture of the objective lens is variable.
One example of a conventional MM-type optical pickup will be described with reference to FIGS. 1 and 2. Two suspension wires 2 extend in parallel with each other in the horizontal direction. One end of each of the suspension wires 2 is connected to an upstanding wall portion of an actuator base 1. Further, a holder 4 is positioned above the actuator base 1 for supporting an objective lens 3, and the holder is connected to each other end of the suspension wires 2. Therefore, the holder 4 is movably supported on the suspension wires 2 in a cantilevered fashion.
Magnets 5A and 5B are fixed to the holder 4. Further, focusing coils 7A 7B and tracking coils 8A 8B are mounted on Yokes 6A 6B which upstand from the actuator base 1. One of the magnets 5A confronts the focusing coil 7A and the tracking coil 8A, and the other magnet 5B confronts the focusing coil 7B and the tracking coil 8B. The combination of the holder 4, the coils 7,8, the magnets 5 and the suspension wires 2 is generally referred to as an actuator.
In operation, predetermined currents are passed through the focusing coil 7 and the tracking coil 8, respectively, to move the holder 4 in focusing direction X and tracking direction Y.
With such an actuator, the suspension wires 2 are formed of extremely flexible materials having low elastic modulus so as to ensure precise movement of the holder 4 in response to the magnetic forces. Stated differently, in the conventional actuator, a sufficiently high magnetic flux density has not been obtainable, and therefore, the suspension wires 2 have had to have extremely high flexibility so that the wires 2 do not restrain movement of the holder 4 in response to the generated magnetic attractive force. Further, the attractive forces act at all times between the magnets 5 and the yokes 6. Therefore, it is necessary to apply a force to the holder 4 in a direction to pull the suspension wires 2 so that the suspension wires 2 will not buckle upon movement of the holder 4. Stated otherwise, in FIG. 2, horizontal attraction force F' (which applies tensile force to the suspension wires 2) imposed on the holder 4 must be greater than another attraction force F directed in the opposite direction to the force F' (the force F applies an unwanted buckling force to the wires 2).
To meet such a requirement, it is proposed to select gap dimensions d, d' between the magnets 5 and the focusing and tracking coils 7,8. That is, the gap distance d' positioned remove from the ends of the wires 2 and defined between the magnet 5A and the coils 7A 8A is smaller than a gap distance d positioned close to the ends of the wires 2 and defined between the other magnet 5B and the other coils 7B 8B. As a result, the attractive force between the magnet 5A and the Yoke 6A and across the gap d' is greater than the attractive force between the magnet 5B and the Yoke 6B and across the gap d.
With such a structure, however, if the gap dimensions d, d' are different from each other, imbalance may occur with respect to operating forces f, f' (see FIG. 2) produced on opposite sides of the holder 4 in the direction of the optical axis of the objective lens 3. More specifically, a force P generated with the gap dimensions d, d' is expressed by: ##EQU1## .mu.: the magnetic permeability, d: the gap dimension,
m.sub.1, m.sub.2 the magnetic field intensity.
Therefore, the difference between the gap dimensions d, d' affects, with its squared value, the operating forces f, f'. Due to the imbalance between the operating forces f, f', the actuator tends to roll when it operates, resulting in a reduction in reproduction efficiency or a servo control failure.
To avoid this drawback, one solution is to remove the yokes 6A 6B so that no attractive forces will be produced between the yokes 6 and the magnets 5. With this arrangement, however, the magnetic flux density generated when currents flow through the focusing and tracking coils 7,8 is reduced to less than 50 to 70% of the magnetic flux density that is available if the yokes 6 exist.
Consequently, a small-size, high-efficiency actuator cannot be realized if the yokes are dispensed with.